Look in the setup menu for the DLP. There should be a setting there in the channel setup area that says "Antenna/Cable". This needs to be changed to cable if it isn't already there. Cable channels use diifferent frequencies than OTA, with the exception of the lowest 2-13 channel numbers.
Are you using the same feed line for both TVs? I know that sounds obvious, but an antenna line looks a lot like a cable line (both RG-6 with F connectors). You may need to trace it back and see that it is connected to the splitter where the cable is coming through.
I know these are obvious, but you need to know that the signal is there at the end of the cable before you start looking for problems in the TV. BTW, the reason the question on the cable system was asked is because many cable companies are converting to an all digital format with no analog channels available. You must have a cablecard or cable box to receive channels on these cable systems. Expect this more and more often as time goes by.